1. Field of the Invention
The present invention relates to an induction sealer and more particularly to an improved induction sealer that utilizes multiple induction heads to provide uniform heating to seal containers and other similar items.
2. Related Art
Induction sealing units for sealing, hermetically sealing or tamper-proof sealing a container with a foil liner are typically included in conveyor systems for high volume applications. These systems usually have flat or tunnel sealing heads mounted above a conveyor, which conveyor carries a plurality of containers to be sealed into proximity with the sealing head. A discussion of the general principles of inductive sealing is disclosed in pending application Ser. No. 09/138,159, the entire contents of which is incorporated herein by reference.
The containers to be sealed are preconditioned to include a foil liner disposed over the opening of the container. Usually, the foil liner is held in place by a screw-on or snap-on cap as is known in the art. Sometimes a wax compound and a paper board portion are included above the foil liner.
Once the preconditioned container is brought within a predetermined distance from the sealing head, a coil within the sealing head produces an electromagnetic field near the foil liner which is disposed within the cap. The electromagnetic flux produced by the field causes current to flow in the foil liner which heat seals the foil liner to the opening of the container, thus sealing the container. The downward force applied by the cap during the sealing operation ensures a proper bond between the foil liner and the opening of the container.
Prior art induction sealing units have aligned the induction heads to be parallel to the direction of travel of the containers to be sealed. For example, see pending application Ser. No. 09/138,159 for a description of an induction sealer which aligns the induction head parallel to the workflow direction. As shown in FIGS. 1 and 2, herein, such a sealer includes a sealing head 1000 arranged so that an elongated portion is aligned with the direction of workflow 1100 of a container 1300. As seen in FIG. 2, the head forms a channel 1200 in a ferrite core 1400 under which container 1300 travels. The containers 1300 thus travel beneath the head 1000 in a channel 1200 created within the ferrite core 1400 which is oriented parallel to the direction 1100 of workflow.
This orientation, while operable, causes certain heating problems. The magnetic field is generally oriented along the ferrite core 1400 causing a cap traveling through the induction sealing unit to experience a substantially continuous heating pattern until exiting the unit. This leads to uneven heating.
In small caps (under 70 mm), this uneven heating pattern does not significantly hamper operation. However, when the cap size is large (usually 70 mm or greater), uneven heating of the foil liner causes sealing failures. As a large cap approaches the induction field, the leading and trailing edges (the edges that approach the field perpendicularly) become hotter than the edges approaching the field in a parallel direction and these “hot spots” continue to heat the entire time the container is passing under the head. If sufficient heat is not generated in the parallel edges, the container is not sealed properly.
To overcome this problem, the field intensity can be increased or the time that the container spends in the field can be increased. In either case, due to the uneven heating effect discussed above, the leading and trailing edges can become too hot and can actually start to burn. This condition leads to improper sealing as well as creating a potential fire hazard.
Another way to overcome this problem is to offset the induction sealing unit at an angle with respect to the direction of workflow. This means that different parts of the cap will encounter varying field intensity since the field is at an angle. While helping, this solution is deficient in at least two respects. First, although no point on the rim of the container experiences constant heating, (depending on the angle of the unit with respect to workflow) regions of the cap still tend to over or under heat. A critical balance must, therefore, be achieved to produce satisfactory sealing. This makes the sealing unit more sensitive to fluctuations in operating parameters such as intensity of the field and time spent in the field.
Second, even though offsetting the induction sealing unit helps to alleviate the problems associated with uneven heating, it complicates the set up and makes operating the unit more difficult. Small variations in the angle can require recalibration. Setup is more complicated since the unit cannot simply be aligned with the workflow. Additionally, because different size caps respond differentially, it may require changing the angle of the induction sealer unit each time a different size cap, seal (e.g., foil seal) or other container is processed.
Thus, there is a need for an induction sealing unit that uniformly heats the lids or other similar items to be sealed, especially when utilizing cap sizes larger than 70 millimeters, which does not require angling even when using different size caps, seals or other containers and is, therefore, easier to install, maintain and operate.